The present invention relates to an electric power steering apparatus using a motor as a source of a steering assisting force.
When a steering member, such as a steering wheel is rotation-operated, a related automotive electric power steering apparatus controls output rotation of an electric motor according to steering torque loaded on the steering member. A rotation of the electric motor is transmitted to a steering mechanism through a reduction gear mechanism to thereby assist the steering mechanism in response to a rotation operation of the steering member. Consequently, labor burden imposed on a driver for steering is reduced.
Another related electric power steering apparatus, which uses a mechanism having, for example, a worm and a worm wheel as the reduction mechanism, has been provided (for instance, JP-A-2000-43739).
Meanwhile, in a case where backlash between the worm and the worm wheel is set to be large so as to reduce transmission resistance, this related electric power steering apparatus has encountered a problem of, for example, rattling noises due to a reverse input from tires when an automobile travels straight on a bad road, such as a gravel road.
Conversely, in a case where backlash is set to be stall so as to suppress rattling sounds, transmission resistance increases. Thus, this related electric power steering apparatus has a problem in that when the state of the automobile is changed from a cornering state to a straight traveling state, the return of a steering wheel is worsened.
From the aforementioned respects, it is very difficult to realize a reduction mechanism that has low transmission resistance and that makes less noise.